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Blower door is 250 cfm less with Radon mitigation on vs off. Why????

hotandhumid | Posted in General Questions on

Appreciate all the bright minds on here who have helped solve “mysteries” with my home. I have another problem hoping to get some ideas on. Blower door diagnostics were done both with and without my radon fan/system running. Radon fan is on home exterior with pipes running underneath 12 mil vapor barrier in conditioned and sealed crawl). We had previously experience significant humidity in crawl space while radon fan was operating. Now ( thanks to the help of everyone on here) we have corrected that problem and the crawl space now stays pretty constant.

My question is: Why are the blower door results 250 cfm lower when the radon mitigation system is on( sub slab/Fan tech HP 220) on, than when it’s off? Blower door test was 3428 cfm with Radon on, 3678 with Radon mitigation off.


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  1. User avatar
    Dana Dorsett | | #1

    It probably means you have a leaks in the vapor barrier &/or slab &/or foundation walls, and that the radon fan is depressurizing the basement, counteracting some of the stack effect contribution.

    The blower door numbers are probably with the house DE-pressurized, no? If you measure it with the house pressurized, turning on the radon fan would likely increase the blower door's cfm number by a similar amount.

    3400 cfm/50 is still fairly significant leakage, and it's probably worth chasing down and remediating some of those leaks.

  2. hotandhumid | | #2

    Thanks Dana,

    Sorry, think I got that backwards. The first blower door test was done with the radon on and measured 3428 cfm. Right afterwards, the radon fan was turned off and the Blower measurement was 3178 cfm. The vapor barrier was just re-sealed with Mastic, and the foundation walls were spray foamed ( closed cell). Could the radon fan be pulling air from somewhere else...attic maybe??? House is very humid when it's humid out too. Appreciate your help-driving me nuts trying to figure this out!!


  3. hotandhumid | | #3

    I forgot to mention the crawl space is no longer humid ( stays 39-40 % RH) since it was re-sealed ( spray foam foundation walls, mastic seal to vapor seams), and does not fluctuate like the house. That's why I was wondering if the radon could be pulling air from the attic and into the house somehow-especially when it's warmer outside?? Right now it's temperate outside ( 55-65 degrees) and HVAC is off-which makes it more humid. Have been having to turn on the AC at times just to keep indoor RH below 54% -even though the indoor temps are 72.

  4. Steve Knapp CZ 3A Georgia | | #4

    Have you inspected the fan and pipe in the attic to verify that everything is connected properly?

  5. hotandhumid | | #5

    Hi Steve,

    I should have mentioned the fan and pipe are on the exterior of the house. It vents several inches above a soffit vent/roof (1st floor).

  6. Steve Knapp CZ 3A Georgia | | #6

    Do you have a sump pump in the crawlspace or cinder block foundation walls?

  7. hotandhumid | | #7


    I have cinder block foundation walls which were spray foamed when the crawl space was encapsulated/conditioned. I don't have a sump pump, and there aren't any water leaks/infiltraions in the crawl ( I regularly check following a heavy rain). The crawl space seems to stay dry with low humidity 39% RH/72 degrees, and doesn't vary too much with the outdoor weather. The interior is the opposite-humidity indoors directly corresponds to exterior humidity/dew points.


  8. hotandhumid | | #8

    I also have a dedicated ducted dehumidifier running in the crawl space. I had the crawlspace vapor barrier resealed with mastic on seams a few weeks ago because the crawl space humidity was hard to keep below 50% even with the dehumidifier running 24-7. Since the crawl was resealed, the humidity stays constant at 40% and the dehumidifier rarely runs. Unfortunately, now the house is more humid than before. I'm thinking the radon fan is having an effect on pressure due to 250 cfm difference during blower door test???? Hoping to solve.


  9. Steve Knapp CZ 3A Georgia | | #9

    I was curious about the foundation since concrete blocks are porous and might be providing a path for air movement.

    I could ask a bunch of additional questions but think you should hire a RESNET consultant to take a look at your crawlspace, attic, and HVAC system.

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